This invention relates to a method of making amorphous semiconductor alloys having improved photoresponsive characteristics and devices made therefrom. The invention more particularly relates to a method of making such alloys and devices by plasma deposition from reaction gases wherein the, plasmas are excited by microwave energy. The invention has its most important application in making improved photoresponsive alloys and devices for various photoresponsive applications including photoreceptive devices such as solar cells of a p-i-n, p-n, Schottky or MIS (metal-insulator-semiconductor) type; photoconducting medium such as utilized in xerography; photodetecting devices and photodiodes including large area photodiode arrays.
Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency (18 percent) crystalline solar cells for space applications. When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially defect-free germanium and particularly silicon crystals, and then turn them into extrinsic materials with p-type and n-type conductivity regions therein. This was accomplished by diffusing into such crystalline material parts per million of donor (n) or acceptor (p) dopant material introduced as substitutional impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type. The fabrication processes for making p-n junction crystals involve extremely complex, time consuming, and expensive procedures. Thus, these crystalline materials useful in solar cells and current control devices are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, and when p-n junctions are required, by doping such single crystals with extermely small and critical amounts of dopants.
These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an impossible economic barrier to the large scale use of the crystalline semiconductor solar cells for energy conversion. Further, crystalline silicon has an indirect optical edge which results in poor light absorption in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the single crystal material is replaced by polycrystalline silicon with cheaper production processes, the indirect optical edge is still maintained; hence the material thickness is not reduced. The polycrystalline material also involves the addition of grain boundaries and other problem defects.
In summary, crystal silicon devices have fixed parameters which are not variable as desired, require large amounts of material, are only producible in relatively small areas and are expensive and time consuming to produce. Devices based upon amorphous silicon can eliminate these crystal silicon disadvantages. Amorphous silicon has an optical absorption edge having properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystalline silicon. Further, amorphous silicon can be made faster, easier in larger areas than can crystalline silicon.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be readily doped to form p-type and n-type materials where p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films are normally four-fold ccordinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making p-n junctions for solar cell and current control device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon and germanium, W. E. Spear and P. G. Le Comber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on "Substitutional Doping of Amorphous Silicon", as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196, 1975, toward the end of reducing the localized states in the energy gap in amorphous silicon or germanium to make the same approximate more closely intrinsic crystalline silicon or germanium and or substitutionally doping the amorphous materials with suitable classic dopants, as in doping crystalline materials, to make them extrinsic and or p or n conduction types.
The reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein a gas of silane (SiH.sub.4) was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and deposited on a substrate at a substrate temperature of about 500.degree.-600.degree. K. (227.degree.-327.degree. C.). The material so deposited on the substrate was an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material a gas of phosphine (PH.sub.3) for n-type conduction or a gas of diborane (B.sub.2 H.sub.6) for p-type conduction were premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used was between about 5.times.10.sup.-6 and 10.sup.-2 parts per volume. The material so deposited including supposedly substitutional phosphorus or boron dopant and was shown to be extrinsic and of n or p conduction type.
While it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of making the electronic properties of the amorphous material approximate more nearly those of the corresponding crystalline material.
The incorporation of hydrogen in the above method not only has limitations based upon the fixed ratio of hydrogen to silicon in silane, but, more importantly, various Si:H bonding configurations introduce new antibonding states which can have deleterious consequences in these materials. Therefore, there are basic limitations in reducing the density of localized states in these materials which are particularly harmful in terms of effective p as well as n doping. The resulting density of states of the silane deposited materials leads to a narrow depletion width, which in turn limits the efficiencies of solar cells and other devices whose operation depends on the drift of free carriers. The method of making these materials by the use of only silicon and hydrogen also results in a high density of surface states which affects all the above parameters.
After the development of the glow discharge deposition of silicon from silane gas was carried out, work was done on the sputter depositing of amorphous silicon films in the atmosphere of a mixture of argon (required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon film. This research indicated that the hydrogen acted as an altering agent which bonded in such a way as to reduce the localized states in the energy gap. However, the degree to which the localized states in the energy gap were reduced in the sputter deposition process was much less than that achieved by the silane deposition process described above. The above described p and n dopant gases also were introduced in the sputtering process to produce p and n doped materials. These materials had a lower doping efficiency than the materials produced in the glow discharge process. Neither process produced efficient p-doped materials with sufficiently high acceptor concentrations for producing commercial p-n or p-i-n junction devices. The n-doping efficiency was below desirable acceptable commercial levels and the p-doping was particularly undesirable since it reduced the width of the band gap and increased the number of localized states in the band gap.
The prior deposition of amorphous silicon, which has been altered by hydrogen from the silane gas in an attempt to make it more closely resemble crystalline silicon and which has been doped in a manner like that of doping crystalline silicon, has characteristics which in all important respects are inferior to those of doped crystalline silicon. Thus, inadequate doping efficiencies and conductivity were achieved especially in the p-type material, and the photovoltaic qualities of these silicon alloy films left much to be desired.
Greatly improved amorphous silicon alloys having significantly reduced concentrations of localized states in the energy gaps thereof and high quality electronic properties have been prepared by glow discharge as fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980, and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, which are incorporated herein by reference, fluorine is introduced into the amorphous silicon semiconductor to substantially reduce the density of localized states therein. Activated fluorine especially readily diffuses into and bonds to the amorphous silicon in the amorphous body to substantially decrease the density of localized defect states therein, because the small size of the fluorine atoms enables them to be readily introduced into the amorphous body. The fluorine bonds to the dangling bonds of the silicon and forms what is believed to be a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than is formed by hydrogen and other compensating or altering agents. Fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen because of its exceedingly small size, high reactivity, specificity in chemical bonding, and highest electronegativity. Hence, fluorine is qualitatively different from other halogens and so is considered a super-halogen.
As an example, compensation may be achieved with fluorine alone or in combination with hydrogen with the addition of these element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages so as to form a silicon-hydrogen-fluorine alloy. Such alloying amounts of fluorine and hydrogen may, for example, be in the range of 1 to 5 percent or greater. It is believed that the new alloy so formed has a low density or defect states in the energy gap than that achieved by the mere neutralization of dangling bonds and similar defect states. Such larger amount of fluorine, in particular, is believed to participate substantially in a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to its other characteristics mentioned herein, is believed to be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. It is believed that fluorine also influences the bonding of hydrogen by acting in a beneficial way to decrease the density of defect states which hydrogen contributes while acting as a density of states reducing element. The ionic role that fluorine plays in such an alloy is believed to be an important factor in terms of the nearest neighbor relationships.
Amorphous semiconductor alloys made by the processes hereinabove described have demonstrated photoresponsive characteristic ideally suited for photovoltaic applications These prior art processes however have suffered from relatively slow deposition rates and low utilization of the reaction gas feed stock which are important considerations from the standpoint of making photovoltaic devices from these materials on a commercial basis. In addition, these processes result in high electron temperature plasmas which produce, during deposition, high densities of ions. The production of these ions results in ion bombardment of the materials or they are being deposited which can result in material damage.
Applicants herein have discovered a new and improved process for making amorphous semiconductor alloys and devices. The inventive process herein provides substantially increased deposition rates and reaction gas feed stock utilization. Further, the process of the present invention results in depositions from plasmas with lower electron temperatures and substantially reduced ion densities and hence, substantially reduced ion bombardment and damage of the deposited materials. Still further, the process of the present invention results in the formation of reactive species not previously obtainable in sufficiently large concentrations with prior art processes. As a result, new amorphous semiconductor alloys can be produced having substantially different material properties than previously obtainable. All of the above results, by virtue of the present invention, in amorphous semiconductor alloys and devices made therefrom having improved photoresponsive characteristics while being made at substantially increased rates.
As disclosed in the aforementioned referenced U.S. Pat. No. 4,217,374, new and improved amorphous semiconductor alloys and devices can be made which are stable and composed of chemical configurations which are determined by basic bonding considerations. One of these considerations is that the material is as tetrahedrally bonded as possible to permit minimal distortion of the material without long range order. Fluorine, for example, when incorporated into these alloy materials, performs the function of promoting tetrahedral bonding configurations. Amorphous semiconductor materials having such tetrahedral structure exhibit low densities of dangling bonds making the materials suitable for photovoltaic applications.
Hydrogen, while smaller than fluorine, is by far less reactive. Hydrogen, as a result, in addition to promoting tetrahedral bonding, also promotes other various possible bonding configurations which can introduce defects into the material. For example, H.sub.2 Si bonds are possible. These bonds are weak bonds which can thermally be broken leaving behind dangling bonds. Also, hydrogen requires rather precise substrate temperature control during deposition to promote the preferred tetrahedral bonding. Therefore, hydrogen in small amounts, in conjunction with fluorine in small amounts should make the optimal amorphous semiconductor alloy. It is not hydrogen as a molecule or fluorine as a molecule, however, which is able to perform these functions. It is atomic hydrogen and atomic fluorine which does. From a chemical point of view in the plasma these species exist as free atoms or free radicals.
In accordance with one preferred embodiment, atomic fluorine and/or hydrogen are generated separately as free radicals and reacted with semiconductor free radicals generated within a plasma sustained by microwave energy. As a result, all of the advantages of separate free radical formation are obtained along with all of the advantages of microwave plasma deposition.
In making a commercial photovoltaic device, it is necessary to provide environmental encapsulation of the devices to prevent undesireable chemical reactions within the device materials due to exposure to chemical species contained in the environment. For example, oxidation of contact materials must be prevented. Customarily, relatively heavy and thick materials such as glass or various organic polymer or plastic materials have been proposed to provide such protection. In accordance with a further embodiment of the present invention, such protection is provided which not only provides the required encapsulation, but additionally light in weight and can be easily incorporated in a manner compatible with the formation of the photovoltaic materials of the devices.